10700
J. Phys. Chem. B 2004, 108, 10700-10710
Photoinduced Electron Transfer in Porphyrin-Oligothiophene-Fullerene Linked Triads by Excitation of a Porphyrin Moiety Takumi Nakamura,† Mamoru Fujitsuka,†,§ Yasuyuki Araki, Osamu Ito,*,† Junya Ikemoto,‡ Kazuo Takimiya,‡ Yoshio Aso,‡,§ and Tetsuo Otsubo*,‡ Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, CREST, Japan Science and Technology Agency, Katahira, Sendai, Miyagi 980-8577, Japan, and the Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan ReceiVed: February 27, 2004; In Final Form: May 7, 2004
Photoinduced charge-separation (CS) and recombination (CR) processes of porphyrin (H2P)-oligothiophene (nT)-fullerene (C60) linked triads (H2P-nT-C60, n ) 4, 8, and 12), which were designed to reveal the function of nT as a molecular wire for the electron-transfer (ET) process, have been investigated by time-resolved fluorescence and absorption spectroscopic methods. After the excitation of the H2P moiety in toluene, 1H2P*-nT-C60 showed predominantly the energy-transfer (EN) process generating H2P-nT-1C60*, whereas the CS process was not observed. The EN rate constant depends on the length of nT. In benzonitrile (PhCN) and o-dichlorobenzene (o-DCB), H2P‚+-nT-C60‚- was produced from 1H2P*-nT-C60. The most energetically stable final CS state was confirmed to be H2P-nT‚+-C60‚-, which was produced by the hole-shift process from H2P‚+-nT-C60‚-. The rate constant for the CS process from 1H2P*-nT-C60 to H2P‚+-nT-C60‚decreased with the length of the nT moiety, indicating that the nT moiety acts as a molecular wire. The small damping factor showed a pronounced solvent polarity effect (0.03 Å-1 in PhCN and 0.11 Å-1 in o-DCB), indicating that a long-range ET process through the nT moiety is feasible in polar solvents. The final CS state (H2P-nT‚+-C60‚-) returned to a neutral triad by the CR process between the vicinal electron (radical anion) and hole (radical cation). The lifetime of the CS state showed the solvent polarity effect ranging from 1.5-2.4 µs in PhCN to 14-27 µs in o-DCB at room temperature. The longest lifetime (27 µs) was observed with H2P-8T‚+-C60‚- in o-DCB. From the temperature dependence of the CR rate constant, the reorganization energies were evaluated to be 0.8-1.1 eV for all triads in PhCN and o-DCB.
Introduction Conjugated nanoscale molecules are useful for the molecular electronic devices such as EL devices,1 photovoltaic cells,2 fieldeffect transistors,3 nonlinear optics,4 and electrical conductors.5 Recently, photoinduced electron-transfer systems with highly efficient charge-separation (CS) and slow charge-recombination (CR) processes have been applied to photovoltaic cells.2 Highly conjugated nanoscale molecular wires are indispensable for single-molecule electron devices.5 Oligothiophenes (nT’s) seem to be one of the most well-investigated oligomers of π-conjugated polymers6-8 because of remarkable characteristics of nT’s such as a rigid rodlike structure and a highly delocalized π conjugation. These nT’s with high π conjugation are expected to serve as molecular wires for electron transfer (ET) and energy transfer (EN). In general, the photoinduced CS process is mainly influenced by redox properties of the acceptor and donor, the reorganization energy, and the distance between the acceptor and donor moieties.9-19 Recently, fullerene C60 has been employed as an electron acceptor of the electron-transfer systems because of its unique characteristics;20 an energy gap of about 1.8 eV separates a 3-fold degenerate LUMO from a lower-lying 5-fold degenerate * Corresponding authors. (O.I.) E-mail:
[email protected]. (T.O.)
[email protected]. † Tohoku University. ‡ Hiroshima University. § Present address: The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan.
HOMO in C60, which is advantageous to attain the appropriate driving force for the CS process via the excited state.21 One of the other remarkable properties of C60 is a low reduction potential (-0.51 V vs SCE, in benzonitrile (PhCN)), affording a good electron-acceptor ability.22,23 Furthermore, another important characteristic of C60 is a low reorganization energy in the electron-transfer processes.15-18,24 Several research groups have reported photophysical and photochemical properties of fullerene-donor linked dyad molecules;25-41 for example, anilineC60,25 carotenoid-C60,26 ferrocene-C60,27 oligothiophene-C60,28 porphyrin-C60,29-33 pyrazine-C60 dyad molecules,34 and so forth.35-41 We recently reported that nT’s mixed with fullerene (C60 or C70) donate an electron to the triplet excited state of fullerene via the intermolecular ET process in polar solvents.42 More recently, efficient CS processes were reported to occur in oligothiophene-C60 dyads.6,28 The lifetimes of the CS states of nT-C60 dyads were on the order of 10-6-10-5 s in polar solvents. It is interesting that the lifetime of the CS state becomes longer for shorter nT’s with a large negative free-energy change (∆G) for the CR process because the CR processes of these dyads are in the Marcus inverted region.43 Furthermore, the CS processes occurred not only in solution but also in fine particles.28c It was also reported that the self-assembled monolayers of these dyads show photovoltaic effects.6 In the present study, we examined the CS and CR processes as well as the EN process in porhyrin-oligothiophene-C60 triads (H2P-nT-C60 (H2P ) triphenylporphyrin in Figure 1)), where
10.1021/jp049122u CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004
Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10701 TABLE 1: Redox Potentials in o-DCBa E/V (vs Fc/Fc‚+) materials H2P-4T H2P-4T-C60 H2P-8T H2P-8T-C60 H2P-12T H2P-12T-C60 H2TPP 4T 8T 12T NMPC60
H2P/H2P
‚+
0.66 0.68 0.55 0.56 0.48 0.49 0.53
nT/nT 0.45 0.43 0.32 0.29 0.22 0.21 0.50 0.34 0.19
‚+
C60/C60‚- H2P/H2P‚- nT/nT‚-1.20 -1.20 -1.19
-1.18
-1.56 -1.58 -1.60 -1.60 -1.62 -1.62 -1.54
-1.59 -1.58 -1.58 -1.58 -1.56 -1.56 -1.59 -1.58 -1.52
a
Figure 1. Molecular structures of H2P-nT-C60 and H2P-nT.
H2P was employed as an antenna molecule harvesting photons in a wide wavelength region because the H2P moiety can absorb visible light more efficiently than the nT-C60 dyads. In these triads, the multifunctional roles of nT’s as a molecular spacer for EN and as a molecular wire for ET were revealed by the time-resolved absorption and fluorescence measurements. To investigate the distance dependence of nT on the efficiencies of the CS and EN processes, we compared three triads by varying the length of nT (n ) 4, 8, and 12). The alkyl groups were introduced as substituents at the β positions of the thiophene rings to obtain a higher solubility of dyad and triad molecules in organic solvents. Experimental Section Materials. Syntheses of H2P-nT and H2P-nT-C60 were described in the previous paper.44 Components such as nT (n ) 4, 8, and 12) and N-methylpyrrolidino-C60 (NMPC60) were prepared according to the methods described in the literature.22a Other chemicals such as tetraphenylporphyrin (H2TPP) and solvents (benzonitrile (PhCN), o-dichlorobenzene (o-DCB), and toluene) were of the best commercial grade available. Methods. Steady-state absorption spectra were measured on a JASCO V-530 UV-vis spectrophotometer. Steady-state fluorescence spectra were measured on a Shimadzu RF-5300 PC spectrofluorophotometer. The fluorescence lifetimes were measured by a single-photon counting method with a streak scope (Hamamatsu Photonics, C4334-01) using the second harmonic generation (SHG, 410 nm) of a Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S, fwhm 1.5 ps) as an excitation source. The nanosecond transient absorption spectra were measured using the pulsed laser light from an optical parametric oscillation (Continuum, Surelite OPO, fwhm 4 ns) pumped by an Nd:YAG laser (Continuun, Surelite II-10). For the measurements of the transient absorption spectra in the near-IR region, we used a Ge avalanche photodiode (APD, Hamamatsu Photonics, B2834) as a detector for monitoring light from the pulsed xenon lamp. The picosecond transient absorption spectra were measured by the pump and probe method using a Ti:sapphire regenerative amplifier seeded by the SHG of an Er-doped fiber laser (ClarkMXR CPA-2001 plus, 1 kHz, fwhm 150 fs). A white continuum pulse light used as a probe light was generated by focusing the fundamental of the amplifier on a rotating H2O cell. The samples were excited by the pulsed laser light from the SHG (388 nm)
Square wave voltammetric measurements were carried out with a potentiostat and a cell equipped with a platinum working electrode, an Ag/Ag+ reference electrode, and a platinum counter electrode in o-DCB containing 0.10 M tetra-n-butylammonium hexafluorophosphate at room temperature.
of fundamentals or output of OPA (Clark-MXR vis-OPA, 560 nm). The probe light that transmitted through the sample in a rotating cell was detected with a dual MOS detector (Hamamatsu Photonics, C6140) for the visible region or an InGaAs linear image sensor (Hamamatsu Photonics, C5890-128) for the nearIR region. Square wave volumetric measurements were carried out with a potentiostat (BAS CV50W) and a cell equipped with a platinum working electrode, an Ag/Ag+ reference electrode, and a platinum counter electrode. All electrochemical measurements were performed in o-DCB containing 0.10 M tetra-n-butylammonium hexafluorophosphate (Nacalai Tesque) at room temperature. The redox potentials were corrected against the ferrocene/ferrocenium (Fc/Fc+) couple. Molecular structures were optimized by the MM3 force field modified in MacroModel package, taking the dielectric constants into consideration. Results and Discussion Optimized Structures. Figure 2 shows the optimized structures of H2P-nT-C60 in polar media, which were calculated by the MM3 force field modified for the MacroModel package, taking the dielectric constant of PhCN ( ) 25.2). It is revealed that the rigid nT chain stretches in a polar solvent and that the H2P moiety and the C60 moiety are spatially separated. In less polar solvents with small dielectric constants (o-DCB ( ) 9.93) and toluene (in vacuum)), similar structures were obtained (Supporting Information, Figures S1 and S2). Steady-State Absorption Spectra. Absorption spectra of H2P-nT-C60 and the corresponding components, H2TPP, nT, and NMPC60, in PhCN are shown in Figure 3. The B band of the H2P moiety in H2P-nT-C60 shifted slightly to the longer wavelength and became broader than that of H2TPP, suggesting that the symmetry of the H2P moiety in H2P-nT-C60 becomes lower than that of H2TPP. These findings indicate a weak interaction between the H2P and nT moieties in H2P-nT-C60. Similar absorption spectra were observed in o-DCB and toluene; no appreciable solvent effect was confirmed in the absorption spectra. Electrochemical Properties and Free-Energy Changes. The redox potentials of H2P-nT-C60 measured by square wave voltammetry in o-DCB are summarized in Table 1. The redox potentials of H2P-8T-C60 (+0.56, +0.29, -1.20 V) and H2P-12T-C60 (+0.49, +0.21, -1.19 V) can be roughly explained on the basis of those of the components, H2TPP (+0.53 V),
10702 J. Phys. Chem. B, Vol. 108, No. 30, 2004
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Figure 2. Optimized structures of H2P-nT-C60 in a polar solvent ( ) 25.2). The structures were calculated by the MM3 method modified for MacroModel.
8T (+0.34 V), 12T (+0.19 V), and NMPC60 (-1.18 V), which imply only a weak electronic interaction among the three moieties. In the cases of H2P-4T-C60 (+0.68, +0.43, -1.20 V) and H2P-4T (+0.66, +0.45 V), an appreciable difference was observed between the H2P moiety and H2TPP, which is due to a positive charge on 4T that reduces the electron density of H2P because the distance between the H2P moiety and the center of the cation radical of the 4T moiety is shorter than those of H2P-8T-C60 and H2P-12T-C60. From the oxidation potentials (Eox) of the H2P and nT moieties, the electron-donor abilities increase in the order of H2P < 4T < 8T < 12T in the dyads and triads. The reduction potentials (Ered) indicate that the electron-acceptor abilities increase in the order of H2P ≈ nT , C60.
The ∆G values for the CS (-∆GCS) and CR processes (-∆GCR) in toluene, o-DCB, and PhCN were calculated using Weller’s equation (eqs 1 and 2):45,46
-∆GCR ) Eox - Ered + ∆GS
(1)
-∆GCS ) ∆E0-0 - (-∆GCR)
(2)
where ∆E0-0 is referred to the energy of the lowest excited state and ∆GS is referred to as a static Coulomb interaction calculated by the ordinary method. (See the equation in the footnote of Table S1 in Supporting Information.)45,46 These ∆GCR values show that the most stable radical-ion pair in the triads is H2P-nT‚+-C60‚- (nT ) 4T, 8T, and 12T). From these ∆GCS and ∆GCR values, the energy diagram can be drawn as shown
Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10703
Figure 4. Schematic main processes for H2P-nT-C60 in polar solvents, considering driving forces (-∆GCS and -∆GEN) and free energy changes (∆GCR). Each energy level varies with nT and solvent polarity. Because energy levels of 3H2P*-nT-C60 are similar to those of H2P-nT-3C60*, they are described as 3[H2P-nT-C60]*.
Figure 5. Fluorescence spectra of H2P-nT (in toluene and PhCN) and H2P-nT-C60 (in toluene and PhCN). Excitation wavelength is 410 nm. Absorption intensities of the samples were matched at the excitation wavelength. Figure 3. Steady-state absorption spectra of (a) H2P-4T-C60, (b) H2P-8T-C60, and (c) H2P-12T-C60 and their components in PhCN.
in Figure 4, in which the energy levels vary with nT’s and solvent polarity. Possible EN, CS, and CR processes after photoexcitation of the H2P and nT moieties are also shown in Figure 4. Steady-State Fluorescence Measurement. Figure 5 shows the fluorescence spectra of H2P-4T-C60 and H2P-4T upon excitation of the H2P moiety with 410-nm light in PhCN and toluene; the absorption intensities of the samples were matched at the excitation wavelength for the fluorescence measurements. The fluorescence band at 670 nm with a shoulder at about 725 nm of H2P-4T can be attributed to the 1H2P* moiety. Because the fluorescence intensity in PhCN was smaller than that in toluene, the CS process between the H2P and 4T moieties takes place. From the ∆GCS values, the formation of H2P‚--4T‚+ via 1H2P*-4T is possible in PhCN (Supporting Information, Table S2). For H2P-4T-C60 in toluene, the fluorescence intensity of the 1H2P* moiety decreased compared with that of H2P-4T, suggesting the EN process to the C60 moiety in H2P-4T-C60 because the CS process to H2P‚+-4T-C60‚via 1H2P*-4T-C60 is not possible because of the positive ∆GCS value in toluene (Supporting Information, Table S1). In
PhCN, a further decrease in the fluorescence intensity was observed compared with that in toluene, suggesting the CS process to H2P‚+-4T-C60‚- via 1H2P*-4T-C60 in addition to the EN process. Such a CS process to H2P‚+-4T-C60‚- via 1H P*-4T-C is possible because of the negative ∆G 2 60 CS value in PhCN (Supporting Information, Table S1). In o-DCB, similar fluorescence quenching behavior was observed, indicating that both the CS and EN processes take place. For H2P-nT and H2P-nT-C60 (n ) 8 and 12), similar tendencies to H2P-4T and H2P-4T-C60 were observed in toluene and PhCN, although it was revealed that the differences depend on the length of the nT moiety. Time-Resolved Fluorescence Measurement in Toluene. Figure 6 shows the fluorescence temporal profiles of H2P-4T and H2P-4T-C60 after the excitation of the H2P moiety in toluene and PhCN; for H2P-nT and H2P-nT-C60 (n ) 8 and 12), similar fluorescence temporal profiles were observed. The profile of 1H P*-nT can be fit with a single exponential from which the 2 fluorescence lifetime (τF) of the 1H2P* moiety was evaluated to be 3.9 ns. Because the τF (toluene) values for 1H2P*-nT are independent of the nT moieties (Table 2), this value (3.9 ns) is a characteristic fluorescence lifetime (τ0) of the H2P moiety without the ET and EN processes in the dyads. Thus, τ0-1 is
10704 J. Phys. Chem. B, Vol. 108, No. 30, 2004
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TABLE 2: Fluorescence Lifetimes (τF), Quenching Rate Constants (kq),a and Quantum Yields (Φq)a toluene ( ) 2.38) materials
τF (ns)
H2P-4T H2P-4T-C60 H2P-8T H2P-8T-C60 H2P-12T H2P-12T-C60
3.9 0.26 3.9 2.8 3.9 3.7
a
kq (s-1)
o-DCB ( ) 9.93) Φq
3.6 × 109
0.94
1.0 × 108
0.28
1.4 × 107
0.04
PhCN ( ) 25.2)
τF (ns)
kq (s-1)
Φq
τF (ns)
kq (s-1)
Φq
3.2 0.21 2.9 1.7 2.8 2.6
5.6 × 107 4.5 × 109 8.8 × 107 3.3 × 108 1.0 × 108 1.3 × 108
0.18 0.95 0.26 0.56 0.28 0.34
3.3 0.18 1.4 0.41 1.3 0.68
4.7 × 107 5.3 × 109 4.6 × 108 2.2 × 109 5.1 × 108 1.2 × 109
0.17 0.95 0.64 0.90 0.66 0.82
The kq and Φq values were calculated from eqs 3 and 4.
TABLE 3: Rate Constants (kEN(P*-C)) and Quantum Yields (ΦEN(P*-C)) for Energy Transfer of H2P-nT-C60 in Toluene, PhCN, and o-DCB solvent toluene PhCN o-DCB
}
initial statea
final statea
1H P*-nT-C 2 60
H2P-nT-1C60*
kEN(P*-C) (ΦEN(P*-C)) kEN(P*-C)b (ΦEN(P*-C))c (ΦEN(P*-C))d (ΦEN(P*-C))d
toluene PhCN o-DCB
n)4 k (s-1)
n)8 k (s-1)
n ) 12 k (s-1)
3.6 × 109
1.0 × 108
1.4 × 107
(0.94) (0.65) (0.76)
(0.28) (0.04) (0.17)
(0.04) (0.01) (0.04)
a The energy difference between 1H P* to 1C * was evaluated to be 0.13 eV from the fluorescence bands. b The values of k 2 60 EN(P*-C) were estimated from the fluorescence quenching rates of 1H2P*-nT-C60 in toluene from eq 4 assuming that the kEN values in toluene are the same as in polar solvents. c The values of ΦEN(P*-C) in toluene were evaluated from eq 4. d The values of ΦEN(P*-C) in polar solvents were evaluated from eq 6.
The short τF values of the triads indicate that the EN process takes place from 1H2P*-nT-C60, producing H2P-nT-1C60* through the nT moieties as in eq 5. kEN(P*-C)
H2P*-nT-C60 98 H2P-nT-1C60*
1
Figure 6. Fluorescence time profiles of H2P-nT and H2P-nT-C60 in the 680-720 nm region in toluene and PhCN. Excitation wavelength is 410 nm.
defined as kd, where kd ) (kic + krad + kisc) and kic, krad, and kisc are the rate constants for the internal conversion, radiative, and intersystem crossing processes (ISC), respectively. In the case of 1H2P*-4T-C60, fluorescence intensity decreased with time according to a single-exponential function, although a relatively small fraction (10-20%) needed the additional function. From the main decay part, a shorter lifetime than that of 1H2P*-4T was calculated, corresponding to the appreciable decrease in the steady-state fluorescence intensity. The τF values of 1H2P*-nT-C60 varied from 0.26 to 3.7 ns depending on the nT’s as listed in Table 2. On the basis of the τ0 value, the rate constants (kq) and quantum yields (Φq) for the fluorescence quenching processes were calculated from the observed τF values using eqs 3 and 4, respectively, as summarized in Table 2.
kq ) τF-1 - τ0-1 -1
Φq )
(τF
The kq and Φq values for H2P-nT-C60 in toluene are attributed to the rate constants (kEN(P*-C)) and the quantum yields (ΦEN(P*-C)) for the EN processes (eq 5). Both kEN(P*-C) and ΦEN(P*-C) values drastically decreased from 3.6 × 109 to 1.4 × 107 s-1 and from 0.94 to 0.04, respectively, by changing from 4T to 12T (Table 3). Thus, the nT moiety acts as a spacer for EN. Because the dependence of kEN(P*-C) on 1/(RDA)6 was confirmed, the Fo¨rster mechanism is indicated for the present EN process, in which long-range EN was confirmed (RDA ) ca. 55.7 Å for H2P-12T-C60).47,48 Indeed, the kEN values calculated from the Fo¨rster theory are comparable with the observed kEN(P*-C) values. (Supporting Information, Figure S2). However, the EN process driven by the Dexter mechanism, which is operative in the short-distance EN process, does not operate to 55.7 Å.49,50 Time-Resolved Fluorescence Measurements in PhCN and o-DCB. In PhCN (Figure 6), a slightly short τF value (3.3 ns) of 1H2P*-4T was observed corresponding to the decrease in the fluorescence intensity compared with that in toluene. According to the Fo¨rster theory, the kEN value depends on the refractive index of the solvent; however, the difference in the refractive indexes is quite small (1.497, 1.528, and 1.552 for toluene, PhCN, and o-DCB, respectively). Thus, the kEN(P* - C) values may be nearly the same in these three solvents. As shown in Table 2, the τF values of 1H2P*-nT-C60 in polar solvents are shorter than those in toluene, suggesting the presence of processes other than EN; thus, the ΦEN(P*-C) values in polar solvents must be decreased by such a competitive process as calculated by eq 6.
-1
- τ0 )
τF-1
(3) (4)
(5)
ΦEN(P*-C) )
kEN(P*-C) τF-1 (polar solvent)
(6)
Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10705
TABLE 4: Driving Forces (∆GCS and ∆GHS)a and Rate Constants (kCS(P*-C) and kHS(P-T)) of H2P-nT-C60 in PhCN and o-DCB n)4 solvent PhCN
o-DCB
initial state 1H
final state H2P‚+-nT-C60‚-
2P*-nT-C60
H2P‚+-nT-C60‚-
H2P-nT‚+-C60‚-
1
H2P*-nT-C60
H2P‚+-nT-C60‚-
H2P‚+-nT-C60‚-
H2P-nT‚+-C60‚-
(kCS(P*-C))b (ΦCS(P*-C))b (kHS(P-T))c (kCS(P*-C))b (ΦCS(P*-C))b (kHS(P-T))c
n)8
n ) 12
-∆G (eV)
k (s-1)
-∆G (eV)
k (s-1)
-∆G (eV)
k (s-1)
0.14
1.7 × 109 (Φ ) 0.30) 3.5 × 107
0.25
1.6 × 109 (Φ ) 0.67) 7.7 × 107
0.33
6.9 × 108 (Φ ) 0.47) 7.0 × 107
1.4 × 108 (Φ ) 0.24) 4.3 × 107
0.24
0.25 0.07 0.16
8.6 × 108 (Φ ) 0.18) 1.1 × 107
0.35 0.17 0.13
0.31
0.13
1.3 × 107 (Φ ) 0.04) 8.6 × 106
∆GCS and ∆GHS were calculated from eqs 1 and 2, in which -∆GS ) (e /(4π0))[(1/(2R+) + 1/(2R-) - 1/RDA)/S - (1/(2R+) + 1/(2R-))/R], where R+ and R- are the radii of the cation and anion, respectively (4T (7.5 Å), 8T (14.5 Å), 12T (21.6 Å), H2P (5.0 Å), and NMPC60 (4.2 Å)). RDA is the center-to-center distance between the donor and acceptor (H2P-4T (12.5 Å), H2P-8T (20.1 Å), H2P-12T (27.6 Å), H2P-4T-C60 (25.5 Å), H2P-8T-C60 (40.4 Å), and H2P-12T-C60 (55.7 Å)). S and R are dielectric constants of solvents used for photophysical studies and for measuring the redox potentials, respectively. b The kCS(P*-C) and ΦCS(P*-C) values were calculated from eqs 11 and 12, respectively. c From Figure 7b. 2
a
The ΦEN(P*-C) values in PhCN and o-DCB are summarized in Table 3. For all nT’s, the ΦEN(P*-C) values considerably decreased with an increase in solvent polarity, suggesting that the CS process becomes predominant in polar solvents. In Table 2, the kq values of 1H2P*-nT in polar solvents are slightly larger than the corresponding kq values in toluene. This finding implies that the slow CS process occurs from the nT moiety to the 1H2P*, producing H2P‚--nT‚+ because this process has negative ∆GCS values in polar solvents (Supporting Information, Table S2). The kCS(P*-T) and ΦCS(P*-T),dyad values were evaluated by applying eqs 7 and 8 to the τF values of 1H P*-nT; the k 2 CS(P* - T) and ΦCS(P*-T),dyad values in polar solvents are the same as kq and Φq for 1H2P*-nT, as shown in Table 2.
kCS(P*-T) ) [τF-1dyad(polar solvent) - τ0-1] ΦCS(P*-T),dyad )
kCS(P*-T),dyad -1
[τF
dyad(polar
solvent)]
(7) (8)
Because the ∆GCS values for H2P‚--nT‚+ via 1H2P*-nT are similar to those of H2P‚--nT‚+-C60 via 1H2P*-nT-C60, we can assume that this type of CS process takes place in the triads at the same rates as in the dyads. The ΦCS(P*-T),triad values can be calculated from eq 9, as listed in Supporting Information, Table S3.
ΦCS(P* -T),triad )
kCS(P*-T),dyad [τF
-1
triad(polar
solvent)]
(9)
The ΦCS(P* - T),triad values are 1300 nm, and those of the C60‚- moiety appeared at 1000 nm, in addition to the absorption bands of the 3H2P* and 3C60* moieties in the region of 700-800 nm in PhCN.28,57 From the ratio of the absorption intensity of the 12T‚+ moiety to that of C60‚-,28 the generation of H2P-12T‚+-C60‚- was confirmed by the spectrum at 250 ns after the photoexcitation. This finding indicates that H2P-12T‚+-C60‚- was generated via H2P‚+-12T-C60‚-; therefore, the hole of the H2P‚+ moiety shifted to the 12T moiety after the nanosecond laser pulse. Similar transient absorption spectra were observed by the selective excitation of the H2P moiety of H2P-4T-C60 and H2P-8T-C60. From the comparison of the absorption intensity of the 4T‚+ moiety (700 and 1140 nm)28,57 and the 8T‚+ moiety (850 and >1300 nm)28,57 with that of the C60‚- moiety, the formations of H2P-4T‚+-C60‚- and H2P-8T‚+-C60‚- are also confirmed by the spectra of the microsecond region. Figure 8b shows the absorption-time profile at 1520 nm of the 12T‚+ moiety, which indicates the hole shift from H2P‚+-12T-C60‚- to H2P-12T‚+-C60‚- as shown in eq 16 at 50 ns. From this rise, the rate constant for this hole-shift process (kHS((P-T) in eq 16) was estimated to be 7.0 × 107 s-1 for H2P-12T-C60. Similar kHS(P-T) values were calculated for H2P-4T-C60 and H2P-8T-C60 as listed in Table 4. An appreciable solvent polarity effect was not found in the kHS(P-T) values. hole shift, kHS(P-T)
H2P‚+-nT-C60‚- 98 H2P-nT‚+-C60‚-
(16)
From the microsecond-region decay profiles shown in Figure 9c and d, which consist of the single-exponential decay, the rate constant for the decay of H2P-12T‚+-C60‚- was calculated. Becuase the nT‚+ moiety and the C60‚- moiety decayed at the same rate, this process can be attributed to the CR process of H2P-12T‚+-C60‚- (eq 17). For all triads, the kCR(T-C) values were calculated to be on the order of 105 s-1 in PhCN as listed in Table 5. In o-DCB, the nanosecond transient spectra were possible to measure because the intensity of the 4-ns laser was much lower than that of the 150-fs laser; the kCR(T-C) values were similarly calculated to be on the order of 104 s-1 for all triads, as listed in Table 5. Note that the kCR(T-C) values are quite small in o-DCB, indicating that the lifetimes (τRIP) of H2P-nT‚+-C60‚- in less polar solvents are longer than 10 µs. It has been reported that both nT‚+ and C60‚- are not reactive to O2;58 however, both decay rates of the 12T‚+ and C60‚- moieties in H2P-12T‚+-C60‚- were accelerated in the presence of O2, as shown in Figure 9c. Similar phenomena were
Figure 9. (a) Nanosecond transient absorption spectra observed at 250 ns (b) and 2.5 µs (O) after 560-nm laser light excitation of H2P-12T-C60 (1.0 × 10-4 M) in Ar-saturated PhCN. (b and c) Absorption-time profile at 1520 nm. (d) Absorption-time profile at 1020 nm.
observed for other H2P-nT‚+-C60‚-. This finding indicates that H2P-nT‚+-C60‚- gains triplet character by mixing with the triplet states of H2P-nT-C60, which have a similar energy to that of
10708 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Nakamura et al.
Figure 10. Modified Arrhenius plots (eq 18) of the temperaturedependent kCR(T-C) for H2P-4T-C60 (b), H2P-8T-C60 (9), and H2P-12TC60 (2).
H2P-nT‚+-C60‚-. This is one of the interpretations for such long τRIP values of H2P-nT‚+-C60‚-. kCR(T-C)
H2P-nT‚+-C60‚- 98 H2P-nT-C60
(17)
To estimate experimentally the energy barrier for the CR step from H2P-nT‚+-C60‚- to H2P-nT-C60, we measured the kCR(T - C) values of H2P-nT‚+-C60‚- as a function of the temperature in PhCN (275-293 K) and o-DCB (265-298 K). The kCR(T-C) values of H2P-nT‚+-C60‚- increased with an increase in the temperature; that is, the kCR(T-C) values in H2P-4T‚+-C60‚changed from 5.0 × 105 to 9.2 × 105 s-1 in PhCN and from 4.9 × 104 to 6.2 × 104 s-1 in o-DCB. Such differences in the kCR(T-C) values are larger than the experimental error in our measurements. In general, the CR rate constant (kCR(T-C)) can be described by the semiclassical Marcus equation (eq 18);43
ln(kCR(T-C)T1/2) ) ln
[
](
)
-∆GCRq 2π3/2V2 + kBT h(λkB)1/2
(18)
Hereby T, h, kB, and ∆GCRq refer to temperature, Planck’s constant, the Boltzman constant, and the Gibbs activation energy in Marcus theory, respectively.43 The λ and V are referred to as the reorganization energy and electron-coupling matrix element for the CR process, respectively. The plots of the observed ln(kCR(T - C)T1/2) values versus 1/T gave the linear relations as shown in Figure 10. From the slope that is equal to -∆GCR(T-C)q/kB, the values of ∆GCR(T-C)q in PhCN were determined to be 0.20, 0.05, and 0.04 eV for H2P-nT‚+-C60‚- (n ) 4, 8, and 12, respectively); in o-DCB, the corresponding ∆GCR(T-C)q values were estimated to be 0.11, 0.02, and 0.01 eV. The ∆GCR(T-C)q values decrease drastically from 4T to 8T. The ∆GCR(T-C)q values in polar PhCN are larger than the corresponding values in less polar o-DCB. The λ values for the CR process were evaluated using eq 1943 by setting ∆G0 ) ∆GCR(T-C), in which the latter values are listed in Table 5.
∆GCR(T-C)q )
(∆G0 + λ)2 4λ
(19)
The calculated λ values in PhCN (0.80, 0.94, and 0.89 eV for H2P-nT‚+-C60‚- (n ) 4, 8, and 12, respectively)) are slightly smaller than the corresponding ones in o-DCB (0.90, 1.20, and 1.10 eV). The λ values are in the order of 4T < 12T e 8T, which suggests that the charge density of H2P-4T‚+-C60‚- is much larger than those in the other triads, whereas the difference
in the charge densities between 8T and 12T is small59 because the reorganization energy due to the solvation factor generally tends to decrease with the larger charged surface area.43 This implies that the hole along the π conjugation in 8T and 12T tends to localize in the middle part of the chain more than that in 4T because the charge per thiophene ring takes a maximum point in the middle of the chain for 8T and 12T.59 A slight increase in the λ values was observed in o-DCB compared to the corresponding values in PhCN, indicating that the interaction between the solvents and charged solute also affects the λ values.43 Because the λ values are smaller than the absolute values of ∆GCR(T-C), the CR processes of H2P-nT‚+-C60‚- occur in the Marcus inverted region. As shown in Table 5, H2P-4T‚+-C60‚has the longest lifetime in PhCN, which is due to the smallest λ value and largest |∆GCR(T-C)|. These factors predict a smaller kCR(T-C) value for H2P-4T-C60 than for H2P-8T-C60 and H2P-12T-C60. A similar trend was observed in o-DCB. The V values for the CR process in PhCN were calculated to be 1.20, 0.14, and 0.11 cm-1 for H2P-nT‚+-C60‚- (n ) 4, 8, and 12, respectively) from the intercept of Figure 10 (the first term of eq 18). The V values for H2P-8T‚+-C60‚- and H2P-12T‚+-C60‚- are extremely small compared to the reported V values (1-100 cm-1) for conventional dyad systems.18,19 Such extremely small V values are the origin of small kCR(T-C) values. The extremely large V value of H2P-4T‚+-C60‚- compared to those of H2P-8T‚+-C60‚- and H2P-12T‚+-C60‚- is also due to the charge density of the nT‚+ moiety. In o-DCB, smaller V values (0.14, 0.02, and 0.02 cm-1 for H2P-nT‚+-C60‚- (n ) 4, 8, and 12, respectively)) were calculated, supporting smaller kCR(T-C) values. Conclusions In polar solvents, the initial CS states were revealed to be H2P‚+-nT-C60‚- produced via 1H2P*-nT-C60, in which the nT moieties act as efficient molecular wires. The ability of molecular wires strongly depends on the solvent polarity; the β value for this CS process was as small as 0.03 Å-1 in PhCN; therefore, the nT’s are superior molecular wires. Then, the hole shift takes place from H2P‚+-nT-C60‚- to H2P-nT‚+-C60‚-; in this process, the nT’s act as hole acceptors. Finally, the CR takes place in H2P-nT‚+-C60‚-, in which the nT‚+ moiety acts as an electron acceptor from the C60‚- moiety. The lifetimes of H2P-nT‚+-C60‚- were longer than 10 µs in o-DCB. In nonpolar solvents, the EN process is competitive with the CS process. In the EN process, the nT’s act as spacers. The lengths and the free-energy changes of the nT’s governed the precedence of the paths. Thus, multifunctional roles of the nT’s in the photoinduced processes of H2P-nT-C60 were revealed in the present study. Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research on Primary Area (417) from the Ministry of Education, Science, Sports, and Culture of Japan and (nos. 10207202, 11740380, and 12875163). We are also grateful to the Mitsubishi foundation. Supporting Information Available: Optimized structures in different solvents, free-energy changes, and driving forces for charge separation, charge recombination, and hole transfer. Rate constants and quantum yields. Calculation of energytransfer rate constants and picosecond transient spectra of reference compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
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